1. Field of the Invention
This invention relates to the impregnation of ceramic monolithic catalyst supports and more particularly to impregnating the support with a predetermined amount of catalyst.
2. The Prior Art
The need to remove or convert the noxious components in vehicular exhaust gases is now well known as a means for overcoming air pollution. Also, the present and proposed future requirements for having catalytic exhaust gas converters on motor vehicles are quite well known. One form in which the catalysts for the converters are supplied is as catalytically coated rigid skeletal monoliths, or honeycomb type of elements which are generally cylindrical or oval in shape, where there are a multiplicity of longitudinal passageways in each unit in order to provide a high surface area.
The rigid, monolithic, skeletal structures are typically made from ceramics which comprise refractory crystalline materials such as sillimanite, magnesium silicates, zircon, petalite, spodumene, cordierite, aluminosilicates, mullite or combinations thereof. Such materials are generally considered to have a porous surface, but to improve the porosity of the surfaces of the skeletal surface, it is generally advisable to provide a highly porous alumina coating over the skeletal structure prior to effecting surface impregnation with a catalytically active material. These monolithic, substantially catalytically inactive monolith skeletal support members have been described in prior art patents, as for example in Keith et al U.S. Pat. Nos. 3,331,787 and 3,565,830, such that it is not deemed necessary to describe them in detail herein.
Typically, and by way of example only, the catalytic component will comprise one or more of the noble and base metals and metal oxides of Groups IB, VB, VIIB and VIII of the Periodic Table, particularly copper, vanadium, chromium, manganese, iron, cobalt, nickel, platinum, palladium, rhodium and ruthenium, with one catalytic metal being used singly or in combination with one or more other active metals.
While various methods are known in the art for coating a monolith support with a refractory coating such as alumina and noble metal catalytic coatings such as platinum, palladium and rhodium, such methods from the standpoint of costs are deficient in minimizing the amount of coating applied, especially when a costly catalytically active precious metal, e.g. platinum, palladium or rhodium is codeposited with the high surface area refractory metal.
Thus, U.S. Pat. No. 3,565,830 discloses the immersion of the monolith skeletal structure in the coating slurry with agitation to coat the internal passageways fully, followed by shaking and gently blowing with air to remove excess coating slurry from the exterior surface and open any plugged passageways. U.S. Pat. No. 3,873,350 to Dwyer et al., also involves immersing the monolith support in a coating slurry, removing the coated support and draining excess slurry while shaking the support, and rotating the support about a substantially horizontal axis while blowing air through the passageways.
Such immersion coating techniques are time consuming, and the slurry coats the entire substrate including the interior skeletal structure as well as the circumferential exterior peripheral surface. When precious metals such as noble metal catalyst salts are incoporated in the slurry, the coating process is unnecessarily expensive. This is because the noble metal coating applied to the circumferential outer surface periphery of the monolith support is unnecessary as no gaseous components to be catalyzed by automotive exhaust gas converters are ever brought into contact therewith during normal use of the catalyzed monolith.
Vacuum pressure impregnation of the ceramic monolith support with the coating slurry has been proposed to speed up the coating process. Thus, in U.S. Pat. No. 4,039,482 a ceramic monolithic member is sequentially coated with alumina and a catalytically active metal such as platinum or platinum wherein the member is placed in a pressure chamber, the chamber flooded with coating slurry and the member vacuum treated to impregnate the member with the slurry. After vacuum impregnation, the slurry is drained from the chamber and pressurized air is blown through the chamber to remove excess slurry from the chamber and the member before the member is subjected to high temperature drying.
In U.S. Pat. No. 4,208,454, the ceramic monolithic member is also flooded with the slurry and the member is subjected to a vacuum to draw slurry through the skeletal passageways of the member. The vacuum application is continued to remove plugging and excess slurry by continuing to draw air through the passageways of the member.
The vacuum impregnation processes of U.S. Pat. No. 4,039,482 and U.S. Pat. No. 4,208,454 both involve flooding the ceramic monolithic member with excess amounts of coating slurry. The handling of large excesses of coating slurry invariably leads to inadvertent by nonetheless costly, loss of high cost catalytic metal material. The flooding of the ceramic member, in the vacuum impregnation process disclosed in U.S. Pat. No. 4,039,482, with coating slurry prior to impregnation still deposits a costly and functionally useless amount of coating material on the exterior peripheral surface of the member.
The use of an excess amount of coating slurry in the processes of U.S. Pat. No. 4,039,482 and U.S. Pat. No. 4,208,454 requires the extra steps of removal of the excess coating material as by air blowing in U.S. Pat. No. 4,039,482 and continued vacuum application as in U.S. Pat. No. 4,208,454. These steps are time consuming and add to the cost of the process.
There is, thus, a need in the art for precisely controlling the amount of alumina and metal catalyst slurries applied to ceramic monolithic catalyst supports to reduce the amount of excess coating required so that there may result an improvement in efficiency of the process and a reduction in coating material loss.